In electrophotographic image forming apparatuses, residual toner remaining on the surface of an image bearing member even after a toner image thereon is transferred onto a recording material or an intermediate transfer medium is removed therefrom using a cleaning device.
Strip-shaped cleaning blades made of an elastic material such as polyurethane rubbers are typically used for such a cleaning device because of having advantages such that the cleaning device has simplified structure and good cleanability. Among such cleaning blades, a cleaning blade in which one end thereof is supported by a supporter, and an edge of the other end is contacted with a surface of an image bearing member to block and scrape off residual toner on the image bearing member, thereby removing the residual toner from the surface of the image bearing member.
In attempting to meet a recent need of forming high quality images, there are image forming apparatuses using spherical toner (hereinafter referred to as polymerization toner), which has a relatively small particle diameter and which is prepared by a method such as polymerization methods. Since such polymerization toner has such an advantage as to have a higher transfer efficiency than pulverization toner, which has been conventionally used, the polymerization toner can meet the need. However, polymerization toner has such a drawback as not to be easily removed from an image bearing member by a cleaning blade. This is because such polymerization toner has a spherical form and a small particle diameter, and easily passes through a small gap between the tip of a cleaning blade and the surface of an image bearing member.
In attempting to prevent polymerization toner from passing through a gap between a cleaning blade and an image bearing member, it is necessary to increase the pressure to the cleaning blade contacted with the surface of the image bearing member to enhance the cleanability of the cleaning blade. However, when the contact pressure of the cleaning blade is increased, the friction between the cleaning blade and the image bearing member is increased, and thereby the tip of the cleaning blade is pulled by the image bearing member in the moving direction of the image bearing member. Specifically, as illustrated in FIG. 8(a), a cleaning blade 62 is pulled by the surface of an image bearing member 123 in a moving direction (indicated by an arrow) of the image bearing member due to increase of friction between the blade and the image bearing member, thereby causing a problem (hereinafter referred to as everted-tip problem) in that an edge 62c of a tip 62a of the blade 62 is everted. In this regard, the thus everted tip has a restoring force, and therefore the tip tends to vibrate, resulting in generation of fluttering sounds.
In addition, when the cleaning operation is continued while the edge 62c of the cleaning blade 62 is everted, a portion of the tip 62a of the cleaning blade 62, which is apart from the edge 62c by few micrometers, is abraded as illustrated in FIG. 8(b). When the cleaning blade 62 is further used for the cleaning operation, the portion of the tip 62a of the blade 62 is further abraded, resulting in lack of the edge 62c of the blade 62 as illustrated in FIG. 8(c). The cleaning blade 62 having no edge cannot remove residual toner from the surface of the image bearing member 123, thereby forming an abnormal image in which background thereof is soiled with residual toner.
In attempting to prevent occurrence of the problem, there is a proposal in which a cover layer made of a resin, which is harder than a rubber and has a pencil hardness of from B to 6H, is formed at least on the edge of the tip of a cleaning blade made of a polyurethane elastomer. It is described therein that by forming such a cover layer, friction between the tip of the cleaning blade and a surface of an image bearing member can be reduced while enhancing the abrasion resistance of the cleaning blade. In addition, it is described therein that since the cover layer is hard and is not easily deformed, occurrence of the everted-tip problem can be prevented.
Further, in attempting to prevent occurrence of the everted-tip problem, there is another proposal in which a blade impregnated with an ultraviolet crosslinkable material including a silicone so as to be swelled is exposed to ultraviolet rays so that the surface of the blade is covered with the crosslinked material, which is harder than rubbers and has good abrasion resistance. It is described therein that by covering the surface of the cleaning blade with an ultraviolet crosslinked material having higher hardness than the elastic blade, the abrasion resistance of the cleaning blade can be enhanced. In addition, it is described therein that since the surface portion of the blade is made of a combination of a rubber and a crosslinked material and has higher hardness than a rubber, the hard portion of the blade can be normally contacted with an image bearing member even when the blade is used over a long period of time and the cover layer of the blade is abraded, resulting in prevention of serious abrasion of the blade and generation of fluttering sounds.
However, even when the above-mentioned cleaning blades are used, occurrence of the above-mentioned problems is hardly prevented if images having a high image area proportion (such as image having large solid images) are continuously produced (i.e., if the amount of residual toner on an image bearing member to be removed by the cleaning blade is large). The reason is considered to be as follows.
Specifically, since the blade has a cover layer on the tip thereof or includes a crosslinked material in a surface portion thereof in the longitudinal direction thereof, the elastic property of the rubber of the blade tends to deteriorate. When the elastic property of the blade is deteriorated, the blade cannot be satisfactorily contacted with the surface of an image bearing member (i.e., the pressure of the blade to an image bearing member varies) if the image bearing member is eccentric or the surface thereof is waved. In addition, when images having high image area proportions are continuously produced and a large amount of residual toner is present on the surface of the image bearing member, the large amount of toner is collected at the tip of the blade by being blocked by the blade. In this case, the residual toner at the tip of the blade tends to pass through a relatively large gap formed between a portion of the blade and the surface of the image bearing member, which are contacted with each other at a relatively low pressure due to eccentricity of the image bearing member or waving of the surface thereof, resulting in occurrence of the above-mentioned abnormal image problem.
In the first mentioned cleaning blade, when a thick cover layer made of a material having high hardness is formed, the elastic property of the rubber constituting the blade is deteriorated by the rigid cover layer, and thereby the blade cannot be satisfactorily contacted with the surface of an image bearing member (i.e., the pressure of the blade to an image bearing member varies). Therefore, the cover layer is preferably as thin as possible. However, when a thin cover layer is formed, the cover layer is easily worn out and the rubber is exposed after a short period of time, resulting in occurrence of the above-mentioned everted-tip problems.
In addition, the second-mentioned cleaning blade has the following drawback. Specifically, in order to impart the same hardness as that of the cover layer of the first mentioned cleaning blade to the surface of the crosslinked material-impregnated cleaning blade, it is necessary that the blade is impregnated with a large amount of ultraviolet crosslinkable material to such an extent that the surface of the blade is covered with the crosslinkable material. In this case, the ultraviolet crosslinkable material penetrates into an inner portion of the blade. When ultraviolet rays irradiate the blade to crosslink the ultraviolet crosslinkable material, the material-impregnated portion of the blade, which has a considerable thickness, becomes too hard, thereby deteriorating the elasticity of the rubber (blade), resulting in occurrence of the above-mentioned abnormal problem. In contrast, when the blade is impregnated with a small amount of ultraviolet crosslinkable material to maintain the elasticity of the blade, the rubber of the blade is not perfectly covered with the crosslinked material (i.e., the surface portion is constituted of a combination of the rubber and the crosslinked material), and therefore the surface portion has lower hardness than the cover layer of the first-mentioned blade. In addition, friction between the surface of the blade and a surface of an image bearing member cannot be satisfactorily decreased, thereby causing the above-mentioned everted-tip problem.
Because of these reasons, the inventors recognized that there is a need for a cleaning blade which has good abrasion resistance and which can be satisfactorily contacted with a surface of an image bearing member at substantially a constant pressure to satisfactorily perform a cleaning operation without causing the fluttering sound problem and the everted-tip problem.